A major problem associated with the use of an electromagnetic or radio frequency shielding enclosure is the loss of shielding effectiveness at the space between the door providing access to the enclosure and the door frame. Several approaches to providing a continuous electrically conducting medium between the door and the door frame have been used in the past. One such approach involves the use of brass finger stock which may be configured to contact a knife-edge extension of the door, the finger stock being electrically connected to the door frame which is in turn electrically connected to the wall. Another approach involves the use of opposed conductive panels to form a door assembly which slides on rollers within the door frame. Inflatable bladders between the panels of the assembly force the panels away from each other and into contact with the door frame as disclosed in U.S. Pat. No. 3,518,355 to Luce.
Another common approach involves the use of specially prepared gaskets between the door and door frame. A tubular mesh gasket may be flattened between the door and frame by the application of force thus exerting pressure on the gasket and maximizing the electrical conductivity through the door surface, gasket and frame structure (see U.S. Pat. No. 3,055,969 to Schaller). If permanent deformation or flattening of this gasket occurs due to loss of resiliency with continued use electromagnetic radiation may leak through the seal. Yet another approach is illustrated in U.S. Pat. No. 3,507,974 to Clark. Inflatable tubing is used to bend flexible finger stock, which is electrically connected to the door structure, into contact with the frame.
A discussion of the disadvantages inherent in these approaches can be found in U.S. Pat. No. 4,177,353 to McCormack. McCormack proposes to seal the gap around a door, in an electromagnetic shielding enclosure with an inflatable, tubular conductive gasket which is confined within a rigid, conductive channel having a semi-circular cross section and recessed within the frame around the door. The gasket proposed by McCormack has a sleeve or coating of conductive material over the tube. Upon inflation of this gasket the surface thereof is forced into good conductive contact with the interior of the channel in which the gasket is confined. A minor part of the gasket is not confined within the channel and, when the gasket is inflated, emerges through the open portion of the channel and, within the limits of the channel geometry and the gasket material, is capable of extruding beyond the surface of the frame in which the channel is recessed. If the adjacent surface of the door that the gasket is to contact is fairly true and in good alignment with the emergent portion of the gasket and if the adjacent surface is nowhere beyond the reach of that small emergent part of the gasket, it should be possible to make an effective seal across the gap between the frame and the door. However, the geometry of the rigid channel and the characteristics of the gasket material impose narrow limits on the ability of McCormack's seal to accommodate misalignment and variations in the gap between the frame and the door. It appears to be useful only in relatively small shielding enclosures where dynamic conditions of every day use do not work to distort the designed alignment and gap distance between the adjacent members of the shielding enclosure.
While these approaches are somewhat useful, it will be appreciated by one skilled in the art that relatively large and uneven gaps generally exist between the mating surfaces of large doors or between such doors and the adjacent surface of the surrounding frame. These gaps occur because it is extremely difficult to hold manufacturing tolerances accurately over large distances, such as the typical dimensions of the doorways of aircraft hangers which must be sealed against radio frequency leakage without incurring excessive costs. Furthermore, even if dimensions are accurately controlled, the great weight of such large doors will make it difficult to maintain the doors in proper alignment during use. Misalignment will inevitably result in gaps which vary in dimension with position along the edge of the door. If an attempt is made to use precise tolerances, such misalignment may prevent the doors from closing. Thus, attempts to reduce the size of such gaps and to keep them of uniform size so that the prior art electromagnetic seals may be effective would be expensive and involve continuing mechanical maintenance.